Ophthalmometer device having target means independently adjustable while maintaining the corneal image in focus

ABSTRACT

A device for determining the topography of an optic including a support member having a measuring unit mounted thereon. The measuring unit includes an optical system having a target assembly operably associated therewith and mounted in optical alignment with the optical axis of the optical system. Means included to maintain the optic in stationary, aligned relation with the optical axis. The target assembly includes a target member adapted to be illuminated by a source of light to produce an illuminated image on the reflective surface of the optic. The target member is mounted for selective telescopic movement along the optical axis of the optical system for increasing or decreasing the size of the image produced on the reflected surface of the optic to enable measurement of the curvature of relatively larger or smaller areas of the surface thereof.

United States Patent Nupuf Feb. 8, 1972 [54] OPHTHALMOMETER DEVICEHAVING TARGET MEANS INDEPENDENTLY ADJUSTABLE WHILE MAINTAINING THECORNEAL IMAGE IN FOCUS Primary Examiner-David Schonberg AssistantExaminer-Paul A. Sacher AttorneyTeare, Tearc & Sammon [72] Inventor:Joseph S. Nupuf, 1342 Cleveland Avenue, [57] ABSTRACT N.W., Canton, Ohio44703 A device for deterrmnmg the topography of an optic including [22]Filed: Sept. 28, 1970 a support member having a measuring unit mountedthereon. The measuring unit includes an optical system having a target 1A LN 76,171 [2 1 pp assembly operably associated therewith and mountedin optical alignment with the optical axis of the optical system. U.S.Means included to maintain the optic in tationary 356/127 relation withthe optical axis. The target assembly includes a [51] Int. Cl ..A6lb3/10, GOlb 9/00 target member adapted to be illuminated by a Source f lih [58] Field of Search ..351/6, 13, 16, 39; 356/127 to produce anilluminated image on the reflective Surface of the optic. The targetmember is mounted for selective tele- [56] References and scopicmovement along the optical axis of the optical system UNITED STATESPATENTS for increasing or decreasing the size of the image produced onthe reflected surface of the optic to enable measurement of Henker thecurvature of relatively larger or smaller areas of the sur- 1,833,48211/1931 Brierton.... .351/10 f h fl' 2,110,330 3/1938 Freeman... ..147/33,542,458 11/1970 Volk ..351/39 8 Claims, 5 Drawing Figures C 0 i 53 I:1 -14 5| 54 I 8 4 76 44 613 48 A r e5 .w l T 7 H 62 a so O 42; 5 22 a 0TI 52 OPHTHALMOMETER DEVICE HAVING TARGET MEANS INDEPENDENTLY ADJUSTABLEWHILE MAINTAINING THE CORNEAL IMAGE IN FOCUS BACKGROUND OF THE INVENTIONThis invention relates to an ophthalmic device and method fordetermining the optical characteristics of an optic, such as the corneaof an eye, a contact lens or the like, and more particularly relates toa measuring device and method for measuring the curvature of the entireoptical surface of an optic.

As the exposed surface of the cornea of the average eye does not have aconstant curvature from its apex to its limbus, it is advantageous toascertain the variations of such curvature in order to design wellfitting contact lenses. Depending upon the individual eye, the cornea,absent astigmatism, is essentially spherically curved within an area of3 to 4 millimeters which area is known as the optical cap, and then,gradually flattens over the extent of the remainder of its curvature toits limbus.

In the ophthalmometers, sometimes referred to as keratometers, used inophthalmic practice, such as described in U.S. Pat. No. 1,750,931,issued Mar. 18, I930, to Gustav A. H. Kellner et al., a target isprovided which produces an illuminated object adapted to be reflected bythe exposed surface of the cornea of the patients eye. In suchinstruments, there is provided a reflective surface on which the patientfocuses the eye to be examined, and the reflected image which is seenthereon serves as a fixation target to maintain the patients eye in astationary and accurate fixation position throughout the examination.The aforementioned illuminated target provides specifically configuredlight beams which strike the cornea of the eye at spaced points on therespective primary meridians, such as approximately 2% millimetersapart. These lighted images produced on the surface of the cornea of thepatients eye are viewed by the operator through a telescopic opticalsystem so arranged as to derive from the light beams reflected from thecornea, a fixed reference image and two adjustable images. With theophthalmometer in proper adjustment a zone of approximately 2%millimeters, defined by the spacing of these images, is measured. Thiszone, known as the visual apex, is the portion of the eye actually usedin vision. The telescopic optical system is provided with an independentcalibrated adjusting means for adjusting the position of each adjustableimage relative to the reference image to order that each adjustableimage can be brought into optical alignment in a predetermined mannerwith the reference image. In this manner, the chordal distance betweenwhere the respective pair of light beams strike the cornea can bemeasured.

These former instruments have been useful in determining the cornealcurvature surrounding the apical zone of the cornea, but have beenlimited to the measurement of a single fixed zone of predetermineddimension, such as 2% millimeters. It has been found extremely desirableto be capable of measuring a plurality of zones both within andsurrounding the apical zone, such as in the range of l to 6 millimeters.More particularly, it has been found desirable to measure such zones ininfinitessimal incremental steps to give a more exact and completedescription of the optical surface of the cornea to determine whetherthe cornea is developing any abnormalities, such as the pathologicalcondition of kerataconus or irregular cornea. More recently, the needfor such a device has increased substantially to improve the quality ofworkmanship and fitting techniques in the application of contact lenses.As the inside of a contact lens must approximately match the curvatureof the cornea to provide a proper fit and allow for normal interchangeof tear flow, it is essential that numerous zones of curvatures of thecornea be measured to enable an accurate measurement over its entiresurface. In addition, such accurate measurements would be extremelybeneficial in order to verify that contact lenses have been accuratelyfabricated by the manufacturer to the design ordered by the eyespecialist and to ensure that there are no defects, abberations ordistortions of the lenses which could result in damage or impairment tothe eye of the intended user.

SUMMARY OF THE INVENTION The present invention contemplates providing animproved ophthalmic device including an optical system for measuring thetopography of an optic, such as the cornea of an eye, a contact lens orthe like. More specifically, the device comprises a support memberadapted for supporting the optic in accurate alignment with the opticalaxis of the optical system. A target means is adjustably mounted on thesupport member for selective movement along the optical axis relative tothe optic being adapted to produce a variable size image thereon uponmovement relative thereto. A measuring unit is operably associated withthe optical system and the target means for determining the topographyof the optical surface of the optic over an area varying in relation tothe size of the image produced thereon.

The target means comprises an extensible target assembly mounted fortelescoping movement along the optical axis including a selectivelyadjustable means operably connected with the support member for movingthe target assembly relative to the optic. The target assembly includesmire means mounted in predetermined relation with respect to the opticalaxis and moveable with respect to the optic being adapted to project animage onto the optical surface of the optic. The target assembly ismoveable independently of the optical system obviating the necessity ofrefocusing the reflected image after each adjustment of the target meansrelative to the optic. The target assembly include a light sourceadapted to illuminate the mire means, and which is moveable, as a unit,with the mire means to maintain the intensity of illuminationsubstantially constant. The target assembly includes a target memberhaving a plurality of mire elements disposed in predetermined spacedrelation thereon being adapted to produce a mirror image thereof on theoptical surface of the optic, and the target member is moveable alongthe optical axis to move the mire elements toward and away from theoptic to increase or decrease, respectively, the size of the reflectedimage of the mire elements on the optical surface. In addition, thetarget member may comprise a plurality of interchangeable target platesadapted for use one at a time during the operation of the device, andwhich target plate includes mire elements disposed in predeterminedspaced relation thereon with the spacing between the respective mireelements on one of the target plates varying from the spacing of themire elements on the other of the target plates so that the imageproduced varies in size for a given setting of the device uponinterchanging of one target plate with any other of the target plates.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a side elevation view of themeasuring device of the present invention;

FIG. 2 is an enlarged, front elevation view of the device illustrated inFIG. 1 taken along the line 22;

FIG. 3 is an enlarged longitudinal section view taken the lines 3-3 ofFIG. 2; and

FIG. 4 is a fragmentary, partial transverse sectional view of the deviceillustrated in FIG. 3 taken along the line 4-4;

FIG. 5 is a fragmentary, transverse sectional view taken along lines 5-5of FIG. 3.

DESCRIPTION OF THE PREFERRED EMBODIMENTS There is illustrated in FIG. 1,generally at 2, a measuring in strument, such as an ophthalmometer orthe like, used for measuring the topography of an optic 7, such as thecornea of an eye or the like. As shown, the measuring instrumentincludes a support pedestal 4 on which is mounted a measuring unit 6.The measuring unit 6 includes a telescopic optical system, illustratedgenerally at 8 in FIG. 3, for viewing the eye 7 of a patient, and beingof the general type well known in the art and described more fully inUS. Pat. No. 1,750,931 issued to G. A. H. Kellner, et al. The supportpedestal may include a chin rest 10 which is adapted to support thepatients head in a manner so as to align the visual axis of the corneawith the optical axis, such as 12 (FIG. 3) of the optical system. Anadjustable target means 14 is supported adjacent the forward end of themeasuring unit in aligned relation to the optical axis of the opticalsystem, and preferably includes specifically configured mire elements,such as at 17 (FIG. 2) which are adapted to be illuminated by a lightsource, as at 16, in a manner well known in the art. Conventionally,these illuminated images are projected in spaced relation on thereflecting surface of the cornea 3 of the patients eye. An actuatingmechanism is operably associated with the target means 14 forselectively adjusting the distance between the mire elements 17 and thecornea 7 for varying the size and/or spacing of the images reflectedthereon, and which images are viewed through the telescopic opticalsystem by the operator. After each selective adjustment of the targetmeans 14, calibrated knobs 22 and 24 of the optical system may beactuated to bring the images reflected on the surface of the cornea intoaligned relation with one another whereby the radius of curvature of theoptical surface of the cornea may be determined in the manner aspreviously described.

As shown in FIG. 3, the measuring instrument 2 includes a cylindricalsupport sleeve 15 which is journaled for rotation within a collar 25.The collar 25 is movably mounted adjacent the upper end of the supportpedestal 4 being adjustable, such as by means of the adjustment knob 26(FIG. 1) to enable relative movement of the measuring unit 6 toward oraway from the eye of the patient being examined. Similarly, adjustmentknobs 27 and 28 may be provided to enable adjustment of the chin restand support pedestal 4, respectively, to accurately and properly alignthe cornea 7 with the optical axis of the optical system.Conventionally, the device includes a fixation target (not shown) onwhich the patient may fix his sight to accurately and properly align thevisual axis of the cornea 3 in substantially coincident relation withthe optical axis of the telescopic system.

As shown, the target means 14 comprises an extensible housing 29 of anysuitable configuration, but is shown as being generally cylindrical intransverse cross section (FIG. 4) and includes an inner sleeve 30 and amoveable outer sleeve 31 mounted in telescoping relation on the innersleeve 30.

The inner sleeve 30 may be physically connected in generally axialalignment to the support sleeve in any suitable manner, such as byscrews or the like, as at 32, having a central axis thereof coincidentwith the optical axis 12 of the optical system 8. In the form shown, theouter sleeve 31 is disposed in generally concentric relation withrespect to the inner sleeve 30 being adapted for sliding movement withrespect thereto. In the form shown, the inner sleeve 30 is provided withtongues 34 and 36 which are adapted to be disposed in sliding relationwithin grooves 38 and 40 provided adjacent the interior surface of theouter sleeve 31 to secure the latter in relatively fixed angularrelation with respect to the inner sleeve 30.

As shown, a target assembly 42 is mounted adjacent the front end of theouter sleeve. The opposite or rear end of the outer sleeve is providedwith an annular flange 44 which is adapted for abutting engagement witha similarly configured oppositely disposed annular flange 46 which maybe formed as part of the support sleeve 15, and which provides a stopfor limiting rearward movement of the outer sleeve with respect to theinner sleeve. The target assembly 42 preferably includes a target plate48 which is positioned adjacent the forward surface of alight-condensing lens 50.

As shown, the lens 50 may be held in place on one side, such as therearward side, by an annular flange 52 which projects inwardly from theinterior surface of the outer sleeve and on its opposite or front sideby a ringlike cap member 53. The cap member may be detachably mounted onthe forward end of the outer sleeve, such as by threading or the like. Adeformable washer member 51 may be provided being adapted to engage thetarget plate adjacent the marginal edge thereof. By this arrangement,the cap member 53, when in the mounted position, will force the washermember against the target plate 48, and thus, firmly hold the latteragainst the front side of the lens 50. As shown in FIG. 5, the targetplate 48 may include a tab 49 extending radially outwardly therefrombeing adapted for registration with a notch 55 in the outer sleeve 31 toprecisely position the mire elements 17 in the proper horizontal andvertical orientation with respect to the optical system.

The target plate 48 is conventionally made of an opaque material whichis provided with spaced, translucent areas defining the mire elements 17which have a specific configuration and orientation with respect to oneanother, and which, in turn, provide an illuminated object to bereflected on the surface of the patients cornea. In the invention, it iscontemplated that the target plate 48 be removable so that other targetplates having larger or smaller spacing between the mire elements may beutilized to provide a wide range of measurement. For example, one targetplate, such as 48, may be used to enable measurements to be madecovering concentric circular zones or areas having transverse dimensionsranging from approximately I to 6 millimeters when the target plate ismoved, such as a distance A, from the fully retracted position to thefully extended position of the housing, as at 57 (FIG. 1). Whereas, thetarget plate 48 may be replaced by another target plate (not shown)including mire elements thereon having a different size and/or spacingdefining zones having transverse dimensions greater than those definedby the target plate 48. As the reflected image on a surface of thecornea of the patients eye is maintained in a substantially fixedposition with respect to the telescopic portion of the optical system,it is not necessary to refocus, such as by means of the eye piece 54, onthe reflected image for every change of position of the target plateand/or change of the target plate itself.

The light source 16 preferably includes a support receptacle 56 which isdetachably mounted on the outer sleeve 31 being moveable therewith, as aunit, upon movement of the outer sleeve with respect to the innersleeve. The receptacle 56 may be connected to a suitable source ofelectrical energy, such as by the conductor 58, to provide power forilluminating an electrical light bulb 59. The receptacle 56 may beattached to the outer sleeve in any suitable manner, such as by screws60 or the like (FIG. 4) which will enable the receptacle 56 to be easilyand readily dismantled from the outer sleeve to replace the bulb 59, asdesired.

To direct the light rays toward the target plate 48 and illuminate themire element 17, a light-deflecting member 62, such as a mirror, isprovided which may be disposed at an angle, such as 45, with respect tothe optical axis of the optical system. In this manner, the lighteminating from the light source 16 will be deflected toward thecondensing lens 50, and thus, illuminate the mire elements. As shown,the mirror 62 is provided with an opening 63 disposed in generally axialalignment with generally circular openings 65 and 67 in the lens 50 andtarget plate 48, respectively, to enable the operator to view thereflected images on the cornea of the patients eye and to enable thepatient to view the fixation target (not shown) which is mounted in amanner so as to align the visual axis of the cornea in substantiallycoincident relation with the optical axis of the telescopic systemv Theopenings 63, 65 and 67 should have a minimum transverse dimension whenmeasured along any line perpendicular to the optical axis, such as thedimension D (FIG. 3), sufficiently large, such as 20 millimeters orgreater so that the reflected images on the cornea can be clearly andeasily seen. Preferably, the outer sleeve 31 is provided with agenerally circular opening 64 through which the light from the lightsource 16 passes. The inner sleeve 30 is provided with an elongated slot66 which may extend for substantially the full length thereof to enablethe light rays to pass into the interior of the inner sleeve 30.

The mirror 62 is initially disposed within the interior of the innersleeve 30 being supported by the outer sleeve 31. As shown, the mirror62 is supported at its forward raised end by a generally L-shapedsupport element 70 which may be secured to the flange 52, such as bywelding or the like. Similarly, the rearward end of the mirror 62 may besupported by another support element 72 which may be suitably secured tothe outer sleeve, such as by welding or the like. By the aforementionedarrangement, the mirror 62 and light source 16 will move, as a unit,with the outer sleeve 31 so that the intensity of the illumination ofthe mire elements will remain substantially constant regardless of thedistance moved by the outer sleeve.

A bellowslike sealing member 74 may be provided to prevent the escape oflight through the slot 66 upon extension of the housing 29. The sealingmember 74 may be attached at one end to the rearward end of the outersleeve 31, such as by rivets 76, and detachably connected at the otherend to the support sleeve 15, such as by a snap ring 78 or the like.Preferably, the flanges 44 and 46 include generally horizontallyextending outer rims 80 and 82, respectively, which extend in adirection toward one another to define an enclosure for the sealingmember.

The actuating mechanism 18 may comprise a rack and pinion type structureincluding a support bracket 84 which is physically fixed to the innersleeve 15. More specifically, a gear element 86 may be provided havingteeth thereon disposed for engagement with teeth provided in a rod 88.As shown, the gear 86 may be mounted on a shaft 90 which is joumaled forrotation in the support bracket 84. Further, the rod 88 may be mountedin sliding relation at one end within an opening 92 provided in thesupport bracket 84 and at its opposite end by a generally upright arm 94which projects upwardly from the flange 44 whereby upon actuation of therod 88, the outer sleeve will be moved with respect to the inner sleeve.The rod 88 may be connected to the arm 94 in any suitable manner, suchas by snap rings 95 or the like. A suitable control knob 97 may beprovided adjacent one end of the shaft 92 to impart selective movementto the rod 88, and thus, the outer sleeve 31. Preferably, the controlknob 97 is provided with indicia elements, such as 99 (FIG. 4), whichmay be calibrated to indicate the distance B, such as in millimeters, ofthe target plate from the surface of the cornea. By this arrangement,the operator will have sufficient information to readily calculate theradius of curvature of the particular zone being measured in a mannerwell known in the art. As these calculations may be time consuming,common charts may be provided which are precalculated to aid theoperator. As these charts are not necessary to an understanding of thedevice, they will not be described herein. Referring again to FIG. 4, areference member 100 may be provided for reading the setting of thecontrol knob 97. More specifically, the reference member 100 may besuitably attached to the support bracket 84 so as to extend outwardlytherefrom having the upper edge 101 aligned with the indicia elementwhich indicates the distance of the target plate from the surface of thecornea. Therefore, the operator may preset the control knob 97 for agiven size image or may make a series of settings which will providemeasurements covering a wide range of generally concentric areas.

OPERATION in a typical operation of the device, the chin of the patientis placed in the chin holder and the operator adjusts the position ofthe chin holder so as to position the cornea of the pa tients eye inoptical alignment with the optical axis of the optical system 8. Thepatient then views the fixation target so as to align the visual axis ofthe cornea in substantially coincident relation with the optical axis,and thus, maintain the cornea in a substantially fixed position toperform the measuring operation. The operator may then adjust the knob26 to position the target plate 48 with respect to the patients corneato obtain, for example, the smallest desired zone size, such asapproximately 1 millimeter. The operator may then focus the image andmake a reading to determine the radius of curvature of the zone or areabeing viewed. As the size of the reflected image will increase as thetarget plate 48 is moved toward the cornea, the knob 97 may then berotated, to move the target plate independently of the optical system intelescoping relation toward the patients cornea to obtain the nextdesired zone size, for example 2 millimeters. After making a reading inthis position, the operator may continue to take readings at positionscloser to the patients cornea until the maximum desired zone size isreached. it is understood, that the operator may begin taking readingsat the maximum desired zone size and gradually withdraw the target plateaway from the patients cornea to take readings at selected positionsuntil the smallest desired zone size is obtained. As the movement of thetarget plate is independent of the optical system, the reflected imageremains in focus throughout the entire measuring process, and thus, thisobviates the necessity of adjusting the telescope for every movement ofthe target plate.

lclaim:

1. An ophthalmometer device including an optical system for determiningthe topography of the optical surface of an optic comprising,

a support member means for maintaining said optic in sub stantiallystationary alignment with the optical axis of said optical system,

a target means for producing an image on the optical surface of theoptic, means for mounting said target means on said support member forselective axial movement independently of said optical system obviatingthe necessity for refocusing said reflected image after each adjustmentof the target means relative to said optic relative to said optic toproduce a variable sized image thereon upon movement relative thereto,and

a measuring unit operably associated with said optical system and saidtarget means for determining the topography of a zone of the opticalsurface varying in relation to the size of the image produced thereon.

2. A device in accordance with claim 1, wherein said target meanscomprises an extensible target assembly mounted for telescoping movementalong said optical axis.

3. A device in accordance with claim 2, wherein said target assemblyincludes a mire means mounted in predetermined relation with respect tosaid optical axis and moveable with respect to said optic so as toproject an image onto the optical surface of said optic.

4. A device in accordance with claim 1, wherein said target meanscomprises at least one sleevelike member mounted in telescoping relationwith respect to said mea suring unit,

a target member is mounted adjacent the end of said sleevelike membernearest said optic, and

an actuating mechanism is operably connected to said sleevelike memberfor selectively moving said target member relative to said optic to varythe size of the image thereon.

5. A device in accordance with claim 1, wherein said target meanscomprises an extensible housing mounted on said measuring unit,

said housing including a first sleeve mounted in relatively fixedrelation adjacent one end of said measuring unit, and

a second sleeve mounted in generally concentric relation with respect tosaid first sleeve being moveable relative thereto along said opticalaxis in a direction toward and away from said optic.

6. A device in accordance with claim 3, wherein said target assemblyincludes a light source for illuminating said mire means, and

said light source is moveable, as a unit, with said mire means tomaintain a substantially constant intensity of illumination thereof.

7. A device in accordance with claim 1, wherein said target meansincludes a target member having a plurality of mire elements disposed inpredetermined spaced relation thereon adapted to produce a mirror imagethereof on the optical surface of said optic,

7 8 said target member being moveable along said optical axis another onone of said target plates being similar to the to move said mireelements toward or away from said orientation on any other of saidtarget plates, and Optic to y the size of the mirror image of Said mirethe spacing between said mire elements on one of said tarmems F opticalSurfaceget plates being different than the spacing on any other of 8. Adevice m accordance with claim 1, wherein said target plates so that theimage produced on said optic will vary in size for a given setting uponinterchanging on one of said target plates with any other of said targetplates.

said target means includes a plurality of interchangeable target plateshaving a plurality of mire elements disposed in predetermined spacedrelation thereon for producing a mirror image thereof on the opticalsurface of said optic,

the orientation of the mire elements with respect to one 10

1. An ophthalmometer device including an optical system for determiningthe topography of the optical surface of an optic comprising, a supportmember means for maintaining said optic in substantially stationaryalignment with the optical axis of said optical system, a target meansfor producing an image on the optical surface of the optic, means formounting said target means on said support member for selective axialmovement independently of sAid optical system obviating the necessityfor refocusing said reflected image after each adjustment of the targetmeans relative to said optic relative to said optic to produce avariable sized image thereon upon movement relative thereto, and ameasuring unit operably associated with said optical system and saidtarget means for determining the topography of a zone of the opticalsurface varying in relation to the size of the image produced thereon.2. A device in accordance with claim 1, wherein said target meanscomprises an extensible target assembly mounted for telescoping movementalong said optical axis.
 3. A device in accordance with claim 2, whereinsaid target assembly includes a mire means mounted in predeterminedrelation with respect to said optical axis and moveable with respect tosaid optic so as to project an image onto the optical surface of saidoptic.
 4. A device in accordance with claim 1, wherein said target meanscomprises at least one sleevelike member mounted in telescoping relationwith respect to said measuring unit, a target member is mounted adjacentthe end of said sleevelike member nearest said optic, and an actuatingmechanism is operably connected to said sleevelike member forselectively moving said target member relative to said optic to vary thesize of the image thereon.
 5. A device in accordance with claim 1,wherein said target means comprises an extensible housing mounted onsaid measuring unit, said housing including a first sleeve mounted inrelatively fixed relation adjacent one end of said measuring unit, and asecond sleeve mounted in generally concentric relation with respect tosaid first sleeve being moveable relative thereto along said opticalaxis in a direction toward and away from said optic.
 6. A device inaccordance with claim 3, wherein said target assembly includes a lightsource for illuminating said mire means, and said light source ismoveable, as a unit, with said mire means to maintain a substantiallyconstant intensity of illumination thereof.
 7. A device in accordancewith claim 1, wherein said target means includes a target member havinga plurality of mire elements disposed in predetermined spaced relationthereon adapted to produce a mirror image thereof on the optical surfaceof said optic, said target member being moveable along said optical axisto move said mire elements toward or away from said optic to vary thesize of the mirror image of said mire elements on said optical surface.8. A device in accordance with claim 1, wherein said target meansincludes a plurality of interchangeable target plates having a pluralityof mire elements disposed in predetermined spaced relation thereon forproducing a mirror image thereof on the optical surface of said optic,the orientation of the mire elements with respect to one another on oneof said target plates being similar to the orientation on any other ofsaid target plates, and the spacing between said mire elements on one ofsaid target plates being different than the spacing on any other of saidtarget plates so that the image produced on said optic will vary in sizefor a given setting upon interchanging on one of said target plates withany other of said target plates.